An asteroid is a rocky or metallic object that orbits the Sun. Many asteroids are found in the asteroid belt between Mars and Jupiter, though they can exist in other orbits too.
How asteroids differ from planets
Asteroids are much smaller than planets and usually have irregular shapes. They are leftover building blocks from the early solar system.
Asteroid versus comet
A comet contains more ice and dust and can form a glowing tail when it gets near the Sun. Asteroids are generally rockier or more metallic.
Asteroids help scientists study the materials that formed the solar system.